Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Color Rendering Index (CRI Ra) is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower). Sodium lights are used, e.g., to light highways—driver response time, however, significantly decreases with lower CRI Ra values (for any given brightness, legibility decreases with lower CRI Ra). See Commission Internationale de l'Eclairage. Method of Measuring and Specifying Colour Rendering Properties of Light Sources, CIE 13.3 (1995) for further information on CRI.
Many methods are known for allowing a lighting device to be adjustable in color temperature, including using a variable combination of warm white and cool white light sources, using red, green and blue light sources. However, all these methods generally provide low to medium CRI Ra.
Several products available today provide luminaries having variable color temperature (variable CCT). The variation in CCT is typically provided by varying the light output level of different color LEDs, such as red, green and blue (RGB) or RGB and white (RGBW) solid state devices so that the combined light output appears to change color. This can provide accent lighting, e.g., changing lighting scenarios, light colors and control of the CCT of the lighting based on the time of day, the presence or absence of people, and the amount of natural light available.
Techniques have been developed for converting the light emitted from LEDs to colored light for illumination purposes. In one technique, an LED can be coated or covered with a phosphor layer having a phosphor material that absorbs radiation energy in one portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. The color of the emission from the LED can be controlled by the selection of excitation source and phosphor. Various combinations of excitation sources and/or phosphor emissions are possible to provide light of a specific color, including white light.
The total of the light from the combination of the phosphor and the solid state light source provides a color point having a corresponding color coordinates (e.g. x and y on the 1931 CIE chromaticity diagram, or u′ and v′ on the 1976 CIE chromaticity diagram) and correlated color temperature (CCT) and a distance from the blackbody locus. However, a CCT is defined only in the vicinity of the blackbody (a.k.a. Planckian) locus. In luminaries of any targeted CCT, the color point preferably lies substantially on the Planckian locus, and the distance (in the form of a circle) from the blackbody locus value is preferably less than 0.010 du‘v’ from the point on the Planckian locus in the 1976 CIE chromaticity diagram (CIE76 diagram). The variation in CCT can also be described in terms of a MacAdam's ellipse, forming ellipse on both the CIE31 diagram (CIE31) and the 1976 CIE chromaticity diagram (CIE76 diagram). It is preferable that the color point for white light is within a 10 or 7 step macAdam's ellipse of the Black body locus.
Providing for and maintaining a single CCT in a specified range, notwithstanding a variable CCT, becomes increasingly difficult when using multiple phosphor blends, especially those using more than two phosphors. Therefore, making multi-phosphor-adjusted LED based lighting devices with variable CCT values remains elusive.